The cardiovascular system has ten unique characteristics that make it an unusually complicated hydraulic system. Understanding how the cardiovascular system functions requires insight into a larger set of variables than that which governs the function of most pump, pipe, and fluid systems found in the world of man-made machines. The ten unique characteristics peculiar to the cardiovascular system are:

1. The system is a closed circle rather than being open-ended and linear.
2. The system is elastic rather than rigid.
3. The system is filled with liquid at a positive mean pressure ("mean cardiovascular pressure"), which exists independent of the pumping action of the heart.
4. The right and left ventricles, which pump into the same system that they pump out of, are in series with two interposed vascular beds (systemic and pulmonary).
5. The heart fills passively, rather than by actively sucking.
6. As a consequence of the heart's passive filling, the circulation rate is normally regulated by peripheral-vascular factors, rather than by cardiac variables.
7. The flow from the heart is intermittent, while the flow to it is continuous.
8. Normally, there is an excess expenditure of energy by the heart needed for the circulation rate imposed by peripheral vascular regulators ("pump energy excess").
9. Normally, ventricular capacity is in excess of the diastolic filling volume ("pump capacity excess").
10. The slowing effect of any vascular resistance on flow rate depends on its location, with reference to upstream compliance, as well as its magnitude.